Open-source alternatives to enterprise-grade code indexing/RAG systems?

I've been exploring various AI coding assistants (Cursor, GitHub Copilot, Devin, etc.) and noticed they all share a common foundation: sophisticated retrieval-augmented generation (RAG) systems that enable deep understanding of codebases. These systems excel at:

- Rapidly indexing entire codebases

- Semantic search across code snippets

- Contextual ranking of relevant code sections

- Integration with LLMs for enhanced code understanding

While proprietary solutions are abundant, I'm looking for open-source alternatives that could provide similar functionality. Specifically:

- Tools for building and maintaining code indexes

- Systems that can integrate with existing LLMs

- Solutions for semantic code search and retrieval

- Frameworks for contextual code understanding

Has anyone built or worked with open-source tools that could serve as building blocks for such a system? I'm particularly interested in hearing about:

- Real-world implementations

- Performance comparisons with commercial solutions

- Scalability considerations

- Integration challenges

The goal is to understand what's possible with current open-source technology in this space, and potentially contribute to building more accessible alternatives to proprietary systems.